Democratic Rep. Steve Israel announced Friday that he will not launch a primary challenge to Sen. Kirsten Gillibrand — at the request of President Obama.

“The president asked me to continue my leadership in Congress” and “not run for the U.S. Senate this year,” the five-term Long Island congressman said in a written statement.

The field-clearing overture in New York is Obama’s first known foray into the Democrats’ candidate selection process for the 2010 congressional elections, and indicates that the White House is prepared to take an aggressive stand to protect the party’s majorities in Congress.

Israel had been telling colleagues in the House that he would challenge Gillibrand.

He was one of several veteran New York officials passed over in January by Democratic Gov. David A. Paterson in lieu of Gillibrand — who had served just one U.S. House term — for the appointment to fill the Senate seat Hillary Rodham Clinton vacated to become secretary of State. He also was a member of a chorus of Democrats who argued that Gillibrand’s record as an upstate House member put her out of step with the left-leaning Democratic voting base on issues such as gun control and immigration.

But Obama intervened to dissuade him from his planned primary challenge Friday, Israel said.

Israel said he had received ample encouragement to run from across the state. But, he said, “In the interest of providing New York and our country with a united front for progressive change, I have decided ... not pursue a campaign for the U.S. Senate.”

“This is a tough, heartfelt decision for me,” he added.

The announcement drew an immediate — and positive — response from the Democratic Senatorial Campaign Committee.

“ Steve Israel is a terrific congressman, and the people of Long Island are lucky to have him as their fierce advocate in Washington,” DSCC Chairman Robert Menendez said in a statement. Gillibrand, meanwhile, “is working around the clock, traveling the state, delivering for New York.”

Israel was considered by a number of insiders to be the most threatening among a handful of prospective primary challengers to Gillibrand. In addition to Israel, two Manhattan-based officeholders — nine-term Rep. Carolyn B. Maloney and Borough President Scott Stringer — and two from Long Island, seven-term Rep. Carolyn McCarthy and Suffolk County Legislature Majority Leader Jon Cooper, have been seriously considering the race.

Of those, Israel and Maloney are the only two with the demonstrated fundraising ability to run a competitive statewide race.

But some Democrats, most prominently senior New York Sen. Charles E. Schumer , have urged Democrats to rally around Gillibrand, expressing concern that a tough primary could make the seat vulnerable to a Republican takeover bid in 2010.