Mara Liasson

One of the sharpest dividing lines emerging between President Obama and GOP presidential front-runner Mitt Romney is the budget introduced in Congress by Rep. Paul Ryan, R-Wis., with its sharp cuts in domestic spending and lower tax rates.

Both sides see it as a winning issue for the fall campaign. The Obama campaign likes to call it the "Romney-Ryan budget" — and Romney hasn't objected.

On the campaign trail in Wisconsin, Ryan was a constant presence with Romney before that state's April 3 Republican primary, which Romney won.

There's a question whether Rick Santorum will prolong his presidential campaign to finish in Pennsylvania later this month. Mitt Romney, on the other hand, is moving in for the kill, buying $1.8 million of airtime in the state. NPR's Mara Liasson reports on the state of the GOP nominating campaign.

As the Supreme Court gets ready to hear arguments about President Obama's health care law, supporters and opponents are planning a flurry of rallies, press conferences and phone banks to remind people why the law is so great — or so terrible. Republicans have been energized by their desire to see the law repealed, but the issue could be more complicated for the GOP than it seems.

Most of the president's speeches these days focus on jobs or gas prices. But the health care law is his signature achievement, and it always gets a mention at political events.

"Change is health care reform that we passed after a century of trying," President Obama said to cheers and applause from the audience at a recent fundraiser in New York.

Mitt Romney won six of the 10 Super Tuesday contests. Rick Santorum won three while Newt Gingrich won one. Ron Paul was the only candidate who did not win at least one contest.

In the Michigan Republican primary Tuesday, Mitt Romney had a near-death experience, but he squeaked out a narrow victory over Rick Santorum. That, says veteran Republican strategist Ed Rogers, has calmed some of the anxiety in Republican circles about Romney's strength as a general election candidate.

"Mitt Romney did what he needed to do to give more certainty and more clarity to the race. He dodged a bullet; it was an ugly win," Rogers says. "It's not over. Santorum is still very competitive."

What's the best way for Mitt Romney to stop Rick Santorum?

For the answer, we went to someone who has done it before.

Democratic strategist Saul Shorr helped Bob Casey defeat then-Sen. Santorum, R-Pa., in a landslide in 2006. Santorum lost by 18 points.

But Shorr says that was a general election; in a Republican primary, Romney will have a much harder job.

Republican presidential candidate Mitt Romney's losses on Tuesday, while not very meaningful in the race to accumulate delegates, have raised questions once again about his ability to inspire passion from his party's base and about his viability in the general election.

Rival Rick Santorum's victories in Colorado, Missouri and Minnesota dealt a setback, if not exactly a body blow, to Romney — whom Santorum routinely dismisses as a candidate with a big machine but no core.

Former Massachusetts Gov. Mitt Romney's decisive win over former House Speaker Newt Gingrich in Florida returned him to the front-runner's spot in the Republican presidential race. Romney emerged from that battle with his strengths, but also his weaknesses, on full display.

Sometimes hard-fought nominating contests produce a more formidable general-election candidate. That's what happened to Barack Obama in 2008. But Dan Schnur, a former Republican strategist, thinks it's too soon to tell if this Republican primary battle will have the same effect.

Saturday's South Carolina Republican primary may be the last good chance for Massachusetts Gov. Mitt Romney's challengers to stop his march to the nomination. Every election year since 1980, the winner of South Carolina's Republican primary has gone on to win the nomination.

All the Republican presidential candidates are campaigning in South Carolina Thursday because the primary is on Saturday. Many voters have still not made up their minds which candidate to vote for.

Mitt Romney's back-to-back wins give him powerful momentum heading into the next set of GOP contests. Ron Paul came in second in Tuesday's New Hampshire primary followed by Jon Huntsman. A week ago, Romney won the Iowa caucuses.

Mitt Romney, Rick Santorum and Ron Paul were the big winners Tuesday night in the Iowa caucuses. They finished first, second and third respectively. Romney won by the narrowest of margins — eight votes.

The mitts are off, so to speak, in the Republican presidential primary. Mitt Romney, the former front-runner, and his current and most serious rival, Newt Gingrich, are now engaged in an all-out war.

With only a few short weeks until voters in Iowa go to the caucuses, Romney is doing everything he can to stop Gingrich's sudden and surprising rise.

There are many flashpoints between Newt Gingrich and Mitt Romney as they battle for the Republican presidential nomination. Most of them are about character or leadership: Who can beat President Obama? Who's the real conservative?

But Gingrich and Romney do have one big policy difference — and that's on immigration.