DAVID MUIR: We're going to turn to politics this morning. And three states are holding primaries today. And the stakes are high for former Alaska Governor Sarah Palin. She's not on the ticket. But she is throwing her support behind candidates in the race. And the big question this morning, does that endorsement actually help? Senior congressional correspondent Jonathan Karl now in Washington. John, good morning.

JONATHAN KARL: Good morning, David. And today, we'll see how much political clout Sarah Palin has in her own state. She has taken sides in the Republican Senate primary in Alaska, launching a tough attack against her state's Republican incumbent senator. It's momma grizzly versus momma grizzly. Sarah Palin is trying to oust Alaska's Republican Senator Lisa Murkowsi.

Palin has endorsed Murkowski opponent Joe Miller, suggesting that unlike Murkowski, he's tough enough to take on the President.

SARAH PALIN: He's got the backbone to take on Obama's radical agenda. By contrast, Lisa Murkowski has voted with the Democrats more than any Republican up for re-election this year.

KARL: The race is a test of Palin's clout in her own backyard. Palin scored some impressive victories earlier this year in the lower 48. Providing critical endorsements to Nikki Haley for governor in South Carolina, and Carly Fiorina for Senate in California.

But, lately, Palin's been on a losing streak. Over the last five weeks, Palin-endorsed candidates have lost in Georgia, Tennessee, Kansas, Colorado and Washington State.

Palin's candidate in Alaska is a hard-line, Tea Party conservative. In a recent interview on ABC's Top Line, he suggested that unemployment benefits are unconstitutional.

JOE MILLER: The unemployment compensation benefits have got to- first of all, is not constitutionally authorized. I think that's the first thing that has to be looked at. So, I do not favor their extension.

KARL: Miller has also been known to attract assault weapon-baring weapon supporters at his political rallies.

MUIR: And, Jon, while we've been following that race in Alaska, I know you going to be following what's going on in Arizona, too. Senator John McCain up against another Republican, who has carved a position even further to the right.

KARL: That's right. And this has been a tough challenge FOR john McCain against J.D. Hayworth, a former Republican congressman. McCain has spent a staggering $21 million to fend off this Hayworth challenge. But, also important to point out, David, McCain is yet another Sarah Palin-endorsed candidate.

MUIR: $21 Million. More than he spent in any of his Senate campaigns. But, I want to ask you about the stem cell judgment from the federal judge, too, while we have you. It's going to be the big issue in Washington today. Blocking President Obama's executive order last year that had expanded embryonic stem cell research. What does that mean for labs this morning? And what was behind the decision.

KARL: Well, this is a major decision. Scientists are scrambling to figure out what the implications are. But, it effectively puts an end, at least temporarily, to all federally-funded embryonic stem cell research. It is a temporary injunction, David. The judge said he believes as a lawsuit challenging the Obama policy goes forward, that all federal funding of research must stop because he believes there's a good chance that the policy will be overturned by the court.