It's not easy to run for president when you're a senator.

Over the weekend, John Kerry took first place in primaries and caucuses in Michigan, Maine, and Washington state. But he's embarked on a rocky road, trying to win votes while working as a senator. Just look at the bumps he's already encountered -- and what he has ahead of him:

Howard Dean has criticized Kerry for not passing more bills through Congress and chided him for raking in money from special interests, even as the senator denounced them on the campaign trail. Wesley Clark has complained that Kerry supported President Bush's tax cuts, the war in Iraq, and the USA Patriot Act. And those were just the attacks from Democrats.

Republican National Committee Chairman Ed Gillespie has painted Kerry as more liberal than the GOP's favorite whipping boy, Massachusetts Senator Ted Kennedy. And there's more to come: Many of the GOP's big guns -- including some in the Senate -- are holding their fire until Kerry officially wins the nomination.

They're planning to make life for Kerry hell by portraying him as an obstructionist, according to a recent article in The Hill. Republicans made that charge in the 2002 Senate elections, costing Max Cleland of Georgia and others their seats, and regaining a slim Senate majority. As John McCain, a man who knows a thing or two about running for president while serving in the Senate, told The Hill, the GOP already has a "very large dossier" on Kerry.

Senator Rick Santorum predicted that if Kerry is indeed the nominee, "things are going to get very political around here." (As if they aren't that way already. Thirty-nine-hour filibusters on judicial nominees, anyone?) And Republican leaders are likely to schedule key votes at inopportune times for Kerry, putting him in the awkward situation of either not voting or of missing campaign events. As Senator George Allen told The Hill, running for president is "kind of an unexcused absence." Imagine what the Republicans will say if John Edwards is the party's vice-presidential candidate.

Meanwhile, President Bush has been crisscrossing the country, making "official visits" right after the Democratic nomination contests in several states. For example, he went to South Carolina on Thursday after Tuesday's primary last week. These visits, not surprisingly, are aimed at generating good press. Bush, however, has not been criticized much for these blatant public-relations ploys. After all, it's hard to prove that his "official" reason for visiting a state is nothing more than an excuse to make up lost political ground. He can also work with GOP majorities on the Hill to make sure there are no scheduling conflicts.

Staffers on the Kerry campaign, meanwhile, are responding swiftly to criticism, and they're also going on the offensive. On Saturday, for example, they e-mailed supporters a letter from former Senate Majority Leader George Mitchell.

"In the Senate," Mitchell, a Maine Democrat, wrote, Kerry "is a strong and effective leader on both foreign policy and domestic issues." He added that Kerry has been especially active on environmental issues, leading the fight to "reduce acid rain emissions, to strengthen the Clean Air Act, and to protect the Arctic [National] Wildlife [Refuge] from drilling."

Yes, Kerry should try to make it to every vote he can in the Senate. But Republicans are counting on the fact that Democrats will try to delay or block some votes to help him. That would play right into the GOP's hands. That's why Senate Minority Leader Tom Daschle needs to make sure that bills won't be decided by a one-vote margin, so if Kerry misses a vote, he can still say what his position is without his absence affecting the outcome. While tactical games may rev up the GOP's base, such antics aren't likely to influence the swing voters that both candidates need to win.

The good news is that when Kerry has been attacked, he's shown he can respond forcefully. There's no question that Republicans -- both in the White House and on Capitol Hill -- will test that strength repeatedly in the months to come. They'll take Kerry's votes out of context and demand explanations. It's imperative that Kerry let voters know what he was thinking when he voted and not tie himself in knots over the complexities of a 19-year Senate record. (This task would no doubt be easier if Americans better understand Congress and their government overall, but enough wishful thinking.) Most importantly, though, when the GOP tries to distract him, Kerry must keep focusing on Bush and Bush's record. That, after all, is what this election is really about.

Mary Lynn F. Jones is online editor of The Hill.