When Barbara Mikulski stunned the political establishment earlier this month by announcing that her fifth term representing Maryland in the Senate would be her last, it took Representative Chris Van Hollen all of two days to declare his candidacy to replace her. Normally, it wouldn't be surprising that a member of the House would jump at the chance to run for Senate—ambitious politicians have long treated the lower chamber as a pitstop on the way to higher office.

But Van Hollen, a 56-year-old serving his seventh term, is no ordinary congressman; widely respected among Democrats for his acumen on both political and policy matters, he has steadily risen to become the party's budget chief and a member of the leadership team. More importantly, he's become such a close confidant of Nancy Pelosi that the Maryland-born Californian was thought to be grooming Van Hollen to be her successor as minority leader, and potentially, the next Democratic speaker of the House. Pelosi told The New York Times last week she was trying to change his mind about the Senate campaign, apparently to no avail. Now Van Hollen is giving all that up—a safe House seat, a top committee post, a shot at the speakership—to run in a crowded Democratic primary and, if successful, to start all over again in an institution that prizes seniority.

There are some obvious advantages to the Senate: You're one out of 100 instead of 435, it's a statewide position, and it's a better springboard for politicians with their eye on the Oval Office. But is being a rookie senator that much better than holding a powerful position in the House?

Chris Van Hollen (Jonathan Ernst/Reuters)

More than half of those currently in the Senate began their congressional careers in the House, but most of those members served only a few years across the Capitol, and few achieved the influence that Van Hollen presently enjoys. The senior ranks of the House are filled with lawmakers who have declined to run for Senate vacancies in their states. "Most of the time when people get to a senior enough position in the House, they don’t want to give that up for the chance to run for the Senate," said Donald Ritchie, the Senate historian. Still, there are exceptions. Pat Roberts of Kansas served in the House for 16 years before running for the Senate, and he has now stayed so long that in January he became the first lawmaker ever to have served as chairman of the Agriculture Committee in both chambers. Ed Markey represented Massachusetts in the House for 37 years before running in a special election to replace John Kerry, after he became secretary of state in 2013. And Missouri's Roy Blunt rose to become interim House majority leader in 2006. Four years later, after Democrats knocked Republicans out of power, he won a seat in the Senate, where he is now again part of the GOP leadership team.

Both Markey and Blunt ditched the House once their party lost the majority, and that may have factored into Van Hollen's thinking. With Republicans gaining even more seats last November, most political prognosticators see little chance for Democrats to win back the House in 2016, or perhaps for the rest of the decade. So while Van Hollen could have run to succeed Pelosi when she retires, his shot at the speaker's gavel was probably a longer way off. (And there would be others eyeing that post, most notably another top Maryland Democrat, Steny Hoyer.) Democrats are now consigned to minority status in the Senate, too, but because of the rules allowing any member to object to legislation, it's not as frustrating as in the House. "Even in the minority, you have some power," explained one Democrat familiar with Van Hollen's thinking. "The minority in the House blows—everyone knows that."

As the upper house of the legislature, the Senate has been the more prestigious chamber of Congress for most of the country's history. But it wasn't always that way. During the first decades of the republic, "the House was the driving engine of the government," Ritchie said. Henry Clay, the legendary Kentucky orator, was twice elected to the Senate early in the 19th century. He then ran for the House, saying he preferred the "turbulence" of the lower chamber to the "solemn stillness" of the Senate. The turning point came with the Missouri Compromise of 1820 that divided the nation into slave and free states. After that, Ritchie said, the great legislators of the time, Clay included, gravitated toward the Senate, and the path between the chambers has been largely one-way ever since. Ben Cardin, a fellow Marylander, faced a choice similar to that of Van Hollen when he decided to run for Senate in 2006. He had served for 20 years in the House, building up seniority on committees and within the Democratic caucus, when Senator Paul Sarbanes vacated his seat. Cardin told me that while he had loved the House, he had "no second thoughts" about his decision. "It was a rare opportunity, and I knew that it doesn’t come along too often," Cardin said. He explained that it was much easier for a single senator, even in the minority, to become relevant on any subject, to draw national attention to an issue, and to have influence.

In recent years, neither house has been held in high regard by the public, although the Senate has come under particular criticism for its near-constant use of the filibuster and its altogether glacial pace. Yet political observers say it has also changed in other ways that make it more attractive to prospective candidates. A wave of retirements over the last decade has allowed newer members to gain seniority much more quickly. Senator Ron Johnson of Wisconsin, for example, is heading a major committee in his first term, while Cardin is near the top third in seniority after just eight years. And gone are the days when junior members were supposed to be "seen and not heard." Just look at Tom Cotton, the Arkansas Republican who arrived in the Senate two months ago and has already drawn national attention—not all of it positive—for getting 46 of his colleagues to sign a letter addressed to the supreme leader of Iran.

People close to Van Hollen—who was not made available for an interview—say his decision had less to do with the political dynamics of the House than with the possibilities offered by the Senate. "I think he feels like the Senate is a place where he can get more done for the entire state," said one Maryland Democrat. "He’s in a position in the House that a lot of people would love to be in. My sense is he looked at, where could he do more?"

Van Hollen begins the Senate race in a strong position. As a former chairman of the Democratic Congressional Campaign Committee, he has a national fundraising network, and in a rare move, Harry Reid endorsed him just days after he entered the race. He's the top Democrat on the Budget Committee and has served as Pelosi's handpicked negotiator during the big fiscal battles of the last few years. Yet he isn't a lock for the seat. Earlier this week, Representative Donna Edwards announced her candidacy after encouragement from progressive groups and African American leaders in the state. And others could jump in as well. "There’s a high risk to run for the United States Senate. Make no mistake about it, that’s a risk," Cardin said. With two of his colleagues running, he said he's staying neutral in the primary. "They know the risk," he said of Van Hollen and Edwards. "They know they’re giving up a much easier time of being reelected to the House, but they also understand the reward: that the United States Senate is a unique body in the world, and that it gives you an opportunity to do an awful lot in a relatively short period of time."