Democratic Senator Evan Bayh of Indiana will not seek re-election this fall, a Democratic aide said on Monday, potentially adding to President Barack Obama's difficulty in pushing his initiatives through a balky Congress.

Bayh becomes the fourth sitting Democratic senator to decide against re-election -- leaving seats up for grabs in Indiana, Illinois, North Dakota and Connecticut -- and analysts say six more Democratic seats are vulnerable to a strong Republican challenge in November.

Bayh said his passion for public service was undiminished but his desire to do so in Congress had waned.

I love helping our citizens make the most of their lives, but I do not love Congress, Bayh said in a prepared statement obtained by Reuters. I will not, therefore, be a candidate for election to the Senate this November.

Bayh was to make a formal announcement of his decision later in Indianapolis,

With big majorities in both the Senate and House of Representatives, Democrats are still heavily favored to retain control of both chambers in the November but face growing worries about the extent of their possible election losses.

Obama has been trying to salvage his top domestic priorities, healthcare and energy, before they sink into an election-year stall, while also seeking support for a multibillion-dollar jobs bill and financial regulatory overhaul.

Voters in heavily Democratic Massachusetts gave the party a shock last month when their dissatisfaction with the economy, unemployment and Obama's agenda helped fuel Republican Scott Brown unexpected Senate election to fill the seat of the late Democratic Senator Edward Kennedy.

In his statement, Bayh said his decision should not reflect adversely on his fellow senators or on the president.

I look forward to working with him during the next eleven months to get our deficit under control, get the economy moving again, regulate Wall Street to avoid future financial crises and reform education, Bayh said.

REPUBLICAN CHALLENGERS

Bayh was elected to the Senate in 1998 and 2004 and insisted he was confident that he could win a third six-year Senate term if he had decided to do so.

My decision was not motivated by political concern, he said. Even in the current challenging environment, I am confident in my prospects for re-election.

However, he might have faced a formidable challenger in former Republican U.S. Senator Dan Coats. Since his name surfaced earlier this month as a challenger to Bayh, Coats has been hammered by Democrats since then for his work as a lobbyist and for living and voting in Virginia for the past decade.

Other potential Republican candidates include Representative Mike Pence and former Representative John Hostettler.

Bayh was not only a popular senator abut also a former governor of Indiana. He had once been seen as a possible presidential contender and was regularly cited as a potential pick for vice-president.

But he was clearly frustrated at the strong partisan streak in the Senate. Democrats control 59 seats in the chamber while Republicans have 41 and votes regularly break along party lines.

Two weeks ago, the Senate voted down a bipartisan commission to deal wit one of the greatest threats facing our nation: our exploding deficits and debt, Bayh said. Just last week, a major piece of legislation to create jobs -- the public's top priority -- fell apart amid complaints from both the left and right.

As a result, Bayh said he had decided there are better ways to serve my fellow citizens ... than continued service in Congress.

Bayh, 54, is the son of former U.S. Sen. Birch Bayh and was considered a centrist who sought common ground with Republicans, making his decision against running again all the more striking since he had allies on both sides of the floor.

(Additional reporting by Steve Holland and Thomas Ferraro, Editing by Jackie Frank)

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