With former Iowa Gov. Tom Vilsack’s decision today to end his quest for the Democratic presidential nomination, an aide to Sen. Tom Harkin (D-Iowa) shot down speculation that Harkin would not seek re-election and pave the way for Vilsack to run for Senate.

Some Democrats believed that the 67-year-old Senate Agriculture chairman would ultimately forgo a fifth term, but stall long enough to allow Vilsack to switch gears and take his place in the 2008 Senate race.

The theory was that the longer Harkin waited before formally announcing a decision, the less time potential Republican candidates would have to prepare to run for an open seat. Money Vilsack raised for his presidential bid could be transferred to a Senate campaign fund.

But Harkin, through a spokeswoman, again dismissed such talk this afternoon.

After speaking to Harkin, Maureen Knightly, his communications director, said there is “no truth” to the “conspiracy theory.”

“He dislikes this lengthy campaign process,” she said in explaining why he will not just say that he is running again.

“He’s doing everything he can right now to prepare for a run and he’s actively raising money,” Knightly added.

Harkin’s schedule is packed with fundraisers and he is hiring more campaign staff, Democrats in Washington, D.C., and Iowa point out.

He began the year with more than $1.1 million in his war chest, according to his latest campaign finance report.

He raised $363,000 in the last quarter of 2006 and listed five people on his campaign pay roll.

Last month, Harkin told Capitol Hill reporters: “Don’t put a lot of stock in that right now,” when a comment by Democratic Senatorial Campaign Committee Chairman Charles Schumer (N.Y.) about Harkin’s reluctance to announce a re-election bid set tongues wagging about Harkin’s political future.

Harkin refused to announce his re-election plans because modern campaigns are “too long,” he said then.