House Democratic leaders, ignoring Republican claims that they were trying to embarrass President Reagan with a veto confrontation, yesterday cleared a fast track for final congressional approval of emergency farm-credit relief legislation that the administration opposes.

But Senate Republican leaders, who oppose passage of the measure, said they will have the last word. Regardless of what the House does, the GOP leaders said, the Senate will vote to sustain a veto, thereby killing the legislation.

The House and Senate Wednesday approved different versions of the relief legislation, which, under normal procedures, would go to a House-Senate conference committee for resolution of the differences.

But yesterday House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) took the rare step of seeking House approval next week of the Senate's version of the relief bill without a conference. The Rules Committee later paved the way for a vote on Tuesday.

O'Neill did not flinch from the potential showdown with Reagan, whose aides have said the president will veto the farm measure. "Well, we'll give him the opportunity to do it," O'Neill said.

In the Senate, however, Majority Leader Robert J. Dole (R-Kan.), who failed in an attempt to block the legislation Wednesday, said Reagan will win in the end. "We've got more than enough votes to sustain a veto," Dole said.

The House approved the measure 318 to 103, which is more than enough to override a veto, but the two Senate votes on the measure fell short of the two-thirds majority required for a bill to become law without the president's signature.

The legislation, approved amid a lobbying blitz at the Capitol by farmers, bankers and public officials, would liberalize credit terms, increase funding for loan guarantees and speed processing of the aid.

But the Senate bill went farther than the House measure in several ways, including providing $100 million for interest subsidies, and House Republicans argued at the Rules Committee that the House should stick by its version and insist on a conference with the Senate.

A veto would affect not only the farm legislation but also a measure to which it was attached that calls for $175 million in disaster relief for starving residents of African countries.

The House yesterday began what may be another veto confrontation when it passed, 294 to 115, and sent to the Senate a second farm bill that would earmark $1 billion in federal loan guarantees for farmers so indebted they are unlikely to qualify for Reagan's debt-restructuring program. The guarantees would be aimed at farmers with $75 in debt for every $100 in assets.

The measure also would provide $17 million for extra personnel to reduce backlogs of loan applications to the Farmers Home Administration.

Vice President Bush took a slap at the farm relief legislation in a speech to the Texas Legislature yesterday, saying it jeopardized efforts to control federal budget deficits.

Dole made a similar point, saying it was "not the best signal to send," although it was "not fatal" to deficit reduction efforts.

Also yesterday, the White House announced that the administration will extend for 30 days the March 1 deadline for farmers to sign up for the price support program. The postponement was made at the request of Iowa Gov. Terry Branstad (R), who met with Reagan yesterday.

An Agriculture Department spokesman, John McClung, said many farmers in Iowa had been reluctant to sign up for price supports because of uncertainty over whether they could get credit for spring planting. He said the administration had been reluctant to extend the deadline at first, but decided to do so because many farmers were not signing up. McClung said the extension would not add to the cost of farm programs. It will apply nationwide.