CONSERVATIVE HOLDS LEAD IN TEXAS SENATE PRIMARY

Representative Kent Hance, a folksy West Texas conservative Democrat who co-sponsored President Reagan's tax package, held a slim lead for the Democratic nomination to the Senate in a runoff primary in Texas tonight, but the outcome remained in doubt.

Mr. Hance's runoff campaign centered on opposition to proposals to grant amnesty to illegal aliens already in the country and charges that his opponent, State Senator Lloyd Doggett, waged a ''mudslinging'' attack on his voting record on Social Security matters.

Although Mr. Hance led Mr. Doggett with over 90 percent of the vote counted, precinct analysts in both camps said the outcome was too close to call because much of the early vote tabulated was from rural precincts that voted heavily for Mr. Hance. The Vote Tally

With 94 percent of 6,063 precincts counted, Mr. Hance had 444,870 votes, or 51.3 percent, and Mr. Doggett had 422,995 votes, or 48.7 percent.

The winner will face the Republican candidate, Representative Phil Gramm, for the seat being vacated by Senator John Tower, a Republican.

The contest for the Democratic nomination was bitterly fought from the beginning. In the first primary campaign, Robert Krueger, the former Representative who came within one percentage point of defeating Mr. Tower six years ago, was considered the front-runner. Mr. Doggett had been expected to run second and Mr. Hance third.