A 51-year-old private investor has given a liberal fund-raising group $50,000 to defeat conservative Del. John S. Buckley (R-Fairfax) and boost the chances of 12 candidates in the Virginia elections Tuesday.

Patrick Butler, a Minnesota native who recently moved to Alexandria, said yesterday he gave the money because of his long-standing support of the Equal Rights Amendment.

"The ERA is most important thing I have seen in my lifetime," said Butler, who is working in Washington on the ERA staff at the National Organization for Women.

Butler's contribution, which has left Republicans mystified, makes up the bulk of $85,715 raised by Citizens for Better Government, a group of liberals, teachers and ERA supporters who are backing a dozen legislative candidates in Virginia.

The group's biggest effort has been aimed at Buckley, a 27-year-old cousin of conservative columnist William F. Buckley Jr., who caused a stir last summer when he sent out a letter blasting the federal Department of Education as a front for "militant homosexuals, free-love advocates, radical women-libbers and pornography merchants."

The group this week launched a $14,000 radio advertising campaign against Buckley, critizing him for sending out the letter under his legislative letterhead and labeling him as one of the most ineffective members of the House of Delegates.

But Buckley claimed yesterday that the ads are backfiring.

"My name identification has shot up, and the phones at campaign headquarters are ringing off the hooks with people calling to volunteer," he said, "We've even run out of bumper stickers."

"Citizens for a Better Government! What a crock," said Buckley, a freshman legislator running for one of three seats in a northwest Fairfax district. "Why do they use such an innocuous name? Why don't they stand up and say, 'We're the teachers' union and ERA supporters'?"

He predicted that most voters will dismiss "this kind of last-minute, negative stuff."

Buckley, a political consultant, has topped all other Northern Virginia candidates in this year's House races by raising $46,901 in what is turning out to be a war between liberal and conservative money. Among his contributors are the National Conservative Political Action committee, which donated $1,250, the Americans against Union Control of Government, Gun Owners of America, and other conservative groups.

Donna Poll, treasurer of the Citizens group, said it was started last summer by several "independent citizens who want to support quality candidates." She and other members were particularly shocked by Buckley's letter, she said, adding: "Buckley is a darling of the New Right. It's about time the far right got a taste of its own medicine."

Although Poll works with the Fairfax Education Association Political Action Committee, she said yesterday that neither the FEA nor the National Education Association has contributed directly to the fund-raising effort.

The NEA did act as the conduit for the collection of $25,400 in individual contributions of less than $100 each, given through the NEA-ERA Fund for Virginia. Other contributors to Citizens for Better Government include the Clarkston, Mich., Education Association, the Michigan Republican Women's Task Force, the NEA Women's Caucus and the Oklahoma Education Political Action Committee.

In addition to Buckley, Citizens for Better Government also has targeted Del. Lawrence D. Pratt (R-Fairfax), executive director of the Gun Owners of America. In Pratt's case, however, the negative attack is limited to direct mailings.

In other races, the liberal group is supporting candidates "where we can make a difference," said Poll, including several seats already targeted by state and national Republicans. In Northern Virginia, the group has given $3,26l to Kenneth R. Plum, a Democratic candidate from Reston, $600 to Del. James Dillard (R-Fairfax), $2,500 to Del. Dorothy S. McDiarmid (D-Fairfax) and $1,000 to Del. Earl E. Bell (D-Loudoun.)

Fairfax GOP chairman Bill Olson yesterday characterized the campaign against Buckley as an effort by "the liberal fringe of the Democratic Party to save Dorothy McDiarmid's seat by running last-minute negative attacks against one of our best Republicans in the House."

"Actually, it has managed to excite support from Republicans and Independents," said Olson, "In that sense, we appreciate their help."