Many vulnerable Congressional Democrats hope first lady Michelle Obama will campaign for them before November — but they’re not banking on a rally or visit.

“I’ve put in lots of requests,” Sen. Claire McCaskill (Mo.) said in late June. “So far, we haven’t made the cut. But I’ve asked. I’ve repeatedly asked, and I’ll keep asking.”

The popular first lady is fervently
campaigning for her husband’s re-election, holding events every week across the country. She’s proved to be a valuable political asset in his first term — wooing donors and swing voters alike.

But Michelle Obama has not hit the campaign trail for any Congressional candidates yet this cycle, despite frequent visits to states with competitive House and Senate races such as Ohio, Pennsylvania and Illinois.

“She’s a very popular figure,” said former Rep. Tom Davis (Va.), the National Republican Congressional Committee chairman in 2000 and 2002. “She could go into some of these Blue Dog districts. She doesn’t attract the same animosity as” her husband.

The Obama campaign could not yet comment on the first lady’s fall campaign schedule. But Michelle Obama has previously indicated she will limit her campaign activities to three days a week in order to spend time with her children in Washington, D.C.

In 2010, the first lady made a limited number of campaign stops for eight Senate candidates, plus three House candidates in Illinois. She also hosted a fundraising event for the Democratic Congressional Campaign Committee. She did all of these events within three weeks of Election Day.

That hasn’t stopped several Democratic candidates from hoping the first lady will campaign for them.

Sen. Sherrod Brown (Ohio), facing a tough re-election race, said “of course” Michelle Obama would be helpful to his campaign. “We’d love to have both of them in the state,” Brown, who campaigned with the president in Ohio last week, said before the recess.

Democratic Senatorial Campaign Committee Chairwoman Patty Murray (Wash.) added, “I think she’s a great campaigner, and people respond very well to her.”

Several Democratic Senators facing voters this cycle, such as Debbie Stabenow (Mich.) and Bob Menendez (N.J.), said they, too, would welcome a visit from the first lady.

The demand exists, but will the first lady deliver this fall in the middle of her husband’s tough re-election race?

“It’s not like she’s flying across the country to do something,” said one veteran Democratic operative who requested anonymity in order to speak candidly about the White House. “But if she’s campaigning in Cleveland, she might carve out 30 minutes on her schedule for Sherrod Brown.”

Members have attended the first lady’s fundraisers and events in their home states. For example, Sen. Amy Klobuchar joined Michelle Obama at a mid-March fundraiser in Minneapolis to benefit the president’s re-election.

It’s no wonder the first lady is sought-after. Sixty-six percent of voters view the first lady favorably, according to a Gallup poll taken in May. That’s 14 points higher than the favorable rating that voters gave the president in the same survey.

Michelle Obama has campaigned at the Congressional level less frequently than her predecessor, first lady Laura Bush.

In 2004, the last cycle an incumbent president sought re-election, Laura Bush started campaigning for Republicans in the spring. As early as March 2004, she hosted a luncheon for then-Rep. Rob Simmons (Conn.). She reportedly raised more than $15 million for Republicans that cycle.

When President George W. Bush’s approval ratings took a nose dive in the 2006 cycle, Republicans booked his popular wife’s schedule with campaign events.  By May of that year, Laura Bush reportedly headlined about a dozen campaign events for Congressional candidates.

President Bill Clinton and then-first lady Hillary Rodham Clinton campaigned frequently for fellow Democrats and their party, sources said. Unlike the Obamas and Bushes, the Clintons had nearly equal approval ratings on average, according to the Gallup poll.

No hard-and-fast data on Hillary Rodham Clinton’s campaign schedule could be found. But sources recalled she was particularly engaged in campaigning for New York candidates in the last couple of years of her husband’s second term as she mounted her own run for Senate.

“For Bill and Hillary, this was an ongoing thing for them. They were always coming into states and raising money for parties,” said Basil Smikle, a New York Democratic consultant. “This president doesn’t really campaign for party institutions and candidates in the same way the Clintons did.”

It should be noted that Vice President Joseph Biden has kept an active presence on the Congressional campaign trail, filling the White House’s need for a versatile presidential surrogate. On Monday, Biden attended fundraisers for McCaskill in Missouri and Sen. Maria Cantwell in Washington state.

Biden, like Michelle Obama and Laura Bush in 2006, is able to campaign in areas of the country not as hospitable to the president.

It’s also worth noting that the president’s children are some of the youngest to occupy the White House in recent history. Democrats argued the first lady’s time is even more scarce for this reason. And naturally, they said, her priority is to ensure her husband’s re-election.

“Job one is getting the president re-elected,” former Clinton White House senior adviser Doug Sosnik said. “The single most limited resource you have in the White House isn’t money — it’s time.”

But there are some states and races the first lady can cross off a potential campaign calendar.

The president remains highly unpopular in West Virginia, where Sen. Joe Manchin (D) is virtually a lock for re-election this November.

So would Manchin want the first lady to stop by for a campaign visit? When asked this question outside the Senate chamber two weeks ago, Manchin simply smiled, said nothing and allowed the elevator doors to close.

There are still some places even Michelle Obama is not welcome.