WASHINGTON, June 29 — After a string of Republican defections this week — on Iraq, immigration and domestic eavesdropping — President Bush enters the final 18 months of his presidency in danger of losing control over a party that once marched in lockstep with him.

First, two prominent Republican senators broke with the president on Iraq. Then, Mr. Bush’s party abandoned him in droves on the immigration bill, sending the measure to its death in the Senate, despite the president’s fervent lobbying for it.

And when Democrats on the Senate Judiciary Committee voted to issue subpoenas to the White House for documents related to its domestic eavesdropping program, three Republicans, including a longtime loyalist, Senator Orrin G. Hatch of Utah, joined them, and another three did not take a position.

For a president who once boasted that he had political capital and intended to use it, the back-to-back desertions demonstrated starkly just how little of that capital is left. With the nation turning its attention to who will succeed Mr. Bush — and Republican presidential candidates increasingly distancing themselves from him — even allies say it could become increasingly difficult for the president to assert himself over his party, much less force the Democratic majority in Congress to bend to his will.