Why is Bill Clinton back on the campaign trail?

Bill Clinton, popular in territory unfriendly to President Barack Obama, is reprising his role as a super-surrogate for Democrats battling to keep their Senate majority and win other races. In the long run, Clinton could pick up political chits for his wife, Hillary Rodham Clinton, should she run for president in 2016.

The political terrain is rough in these Senate battleground states. Obama's signature domestic achievement, the Affordable Care Act, is unpopular. Obama himself has soft poll numbers. Many Democrats won't appear with the president, even though they'll accept his prodigious fundraising help.

Not so with Clinton, who appears Tuesday in Louisville, Ky., with Alison Lundergan Grimes, the Democrat who's trying to unseat Senate Republican leader Mitch McConnell. It's perhaps the nation's hardest-fought Senate race, in a state where Obama would be of little help.

Clinton is the last Democratic presidential candidate to carry a swath of Southern states crucial to the 2014 midterms, including his native Arkansas, Georgia, Kentucky and Louisiana. The former president remains in heavy demand as a fundraiser and adviser as his wife plans an upcoming book tour and considers how she may help Democrats this year.

"He has an open invitation from me," Ed FitzGerald, a Democrat challenging GOP Ohio Gov. John Kasich, said of Clinton.

The Grimes fundraiser marked Clinton's first 2014 campaign event and underscored the party's interest in defeating the Republican leader. The former president personally encouraged the 35-year-old secretary of state to challenge McConnell, a fundraising powerhouse renowned for relentless attacks on his opponents.

Clinton's connection to Grimes goes back a generation: Her father, Jerry Lundergan, is a former state party chairman and longtime Clinton friend. In 1993, as a 14-year-old, Grimes handed the president-elect a bouquet of roses while attending his inaugural festivities.

Clinton is expected to play outsized roles in other Senate races where Democrats are facing steep political odds.

The former president is busily involved in politics on his home turf, where he served five terms as governor and keeps an apartment at his presidential library.

Arkansas Democratic Sen. Mark Pryor is facing a serious challenge from GOP Rep. Tom Cotton. James Lee Witt, Clinton's head of the Federal Emergency Management Agency, is running for Cotton's House seat and will join Clinton at an event in Hot Springs, Ark., on April 5. And Pat Hays, a former North Little Rock mayor who supported Clinton's 1992 campaign, is running for another congressional seat.

Clinton vouched for former Rep. Mike Ross, his one-time gubernatorial campaign driver who is running for governor. "Take it from someone who knows a thing or two about being governor of Arkansas: Mike Ross will be a great one," Clinton wrote in a recent email. Ross could face Asa Hutchinson, a former congressman who was a House manager during Clinton's impeachment trial in 1998.

"He will call about political issues, he will call about candidates, he will call about races, he will call about old friends who have done something or died," said Arkansas Gov. Mike Beebe. Clinton recently phoned from Haiti for an update on the state's plan to use Medicaid funds to buy private insurance for the poor, Beebe said.

Republicans, acutely aware that the Clintons loom large over the nation's political future, are taking note.

Prominent members of the GOP have begun reminding voters about the former president's past. Kentucky Sen. Rand Paul, who is considering a 2016 White House run, recently invoked Clinton's relationship with Monica Lewinsky as "predatory behavior" and said Democrats should scrutinize the former president.

Sticking closer to state politics, McConnell's team likened Grimes to Kentucky Democrats Bruce Lunsford and Jack Conway. Both received Clinton's help in past elections and lost.

But Clinton could be helpful in Senate races in states like Louisiana, where Sen. Mary Landrieu faces a tough re-election campaign, and Georgia, where Michelle Nunn, the daughter of former Sen. Sam Nunn, is running. Her father was among the first Senate Democrats to endorse Clinton in December 1991. Democrats in frequent presidential battleground states like Ohio, Michigan, Pennsylvania, and Florida also expect to see the former president.

While Clinton does not yet have an extensive list of scheduled events for Democrats, party strategists note that he has helped longtime friends and Democrats in past elections.

Clinton's speech at 2012 Democratic National Convention generated headlines for making the case for Obama's re-election, and he joined Obama on the campaign trail. But the former president also helped Senate candidates in nine states, including races in Republican-leaning states like Arizona, North Dakota and Indiana.

Hillary Clinton's political future may benefit from her husband's politicking, observers say, but it's not necessarily why he's helping Democrats this year.

"If she was running or not running, he'd be doing the same thing," said Ira Leesfield, a Miami attorney and Democratic fundraiser who saw the Clintons recently.

And many candidates are happy for the help.

Steven Fulop, mayor of Jersey City, N.J., welcomed the ex-president to a local restaurant in early December for a fundraiser that brought in about $600,000. The 36-year-old Fulop, who supported Hillary Clinton's 2008 presidential campaign, was elected mayor in May 2013 but racked up campaign debts of about $250,000. After the Clinton event, his campaign committee was back in the black.

"He's probably the only person who can get that many people and significant donors in a room at 2 o'clock in December on a cold day," Fulop said.

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