The president has to quit after eight years in the White House, so why do members of Congress get to stick around?

Rand Paul thinks they shouldn't be able to. The Republican senator from Kentucky has long called for term limits for legislators and did so again in formally announcing his presidential campaign Tuesday. It's not likely that the idea would go anywhere, even if Paul beats his GOP rivals and goes on to win next year's election. Still, it's an interesting proposal for what it reveals about Paul as a candidate and a politician.

See Rand Paul’s campaign before he dropped out of the 2016 race

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FILE - In this Feb. 1, 2016 file photo, Sen. Rand Paul, R-Ky, speaks to supporters with his wife Kelley by his side, during a caucus night victory party at the Scottish Rite Consistory in Des Moines, Iowa. Paul is dropping out of the 2016 race for president. A campaign spokeswoman confirmed his decision Wednesday, Feb. 3, 2016, to The Associated Press, saying a statement would be forthcoming. (AP Photo/Nati Harnik, File)

"I ran for office because we have too many career politicians. I believe it now more than ever," Paul said in Louisville Tuesday. "We limit the president to two terms. It’s about time we limit the terms of Congress!"

Term limits were widely debated two decades ago. Eighteen states set limits on legislative terms between 1990 and 1994, according to the National Conference of State Legislatures, and limits are now in effect in 15. The idea was popular with voters -- in most states, substantial majorities of 60 percent or more approved of term limits. And Newt Gingrich, the former Speaker of the House, made a constitutional amendment to limit congressional terms part of the proposed Contract with America in 1994. The amendment did not succeed.

At a basic level, a lawmaker who is not worrying about reelection should be free to do what she thinks is right. Ideally, he or she can spend her time learning about the issues instead of fundraising. He or she can take unpopular votes.

Proponents hope that limiting legislators' terms will also limit pork-barrel projects. Instead of seeking special spending in their districts from the state treasury in order to win votes, lawmakers would be free to vote according to their consciences, or in the best interest of the state as a whole.

While giving individual lawmakers more freedom, term limits also could reduce the power of the political parties. With only a few years in the legislature, the parties' leaders couldn't build up the networks of seniority and patronage that allow effective career lawmakers in each party to enforce discipline within the caucus.

Many voters detest pork and would like to see more competition for Republicans and Democrats, so the broad appeal of term limits isn't surprising. They're especially logical for the tea-party voters who supported Paul during his election to the Senate in 2010. The perceived squishiness of the GOP leadership was partly what incited the tea-party movement, and term limits are, ideally, a way to ensure that new lawmakers can hold to their beliefs, instead of being forced to compromise by a senior legislator who has been in office for the past three decades.

Paul's own career offers a good example. To win his Senate race, he first had to win in the primary against the candidate endorsed by Mitch McConnell, the Republican leader and Kentucky's senior senator. When McConnell was up for reelection, Paul supported him against his tea-party opponent. Any other choice for Paul would have been a strategic blunder, given the power McConnell has accumulated over his 30 years in the Senate.

The problem is that it isn't clear whether term limits actually works in practice, said John M. Carey, a professor at Dartmouth College who has studied the issue. He said researchers have "debunked" the idea that limiting lawmakers' terms will encourage people who aren't set on working as politicians for the rest of their careers to run for office.

"It's a myth," he said, pointing to the example of the Mexican Congress, whose members serve limited terms. Party discipline is uniform there, and the leadership is very powerful. According to Carey, that's because lawmakers rely on their party's leaders to help them find other political work after their terms end.

"The idea that you're going to have a bunch of guys that lay down their plough and then go and serve for a while," Carey said, "was probably wishful thinking in the Roman Republic, let alone now." The problem is that running for office is demanding, and few people do it unless they're willing to devote their entire lives to politics.

One apparent consequence of term limits, Carey said, is that they expand the power of the executive. Without the expertise and the friendships that legislators establish over years of working together, they're more dependent on career public servants for basic information, and they're less able to effectively resist the governor's agenda if they disagree with him or her. In the context of Congress, term limits would increase the influence of the White House and the federal bureaucracy on a day-to-day basis.

It's a conclusion that might give conservatives pause. Paul, for example, has repeatedly suggested that the presidency has too much power, particularly under the Obama administration.