THE TRULY beady-eyed Reaganologist could discern a political signal of some importance from the Justice Department announcement that it had filed its first lawsuit seeking to enforce the Voting Rights Act earlier this week. Although the president has yet to declare whether he favors extending the statue and, if so, whether in its present form or with potentially hobbling amendments, so far the issue of opposition to voting rights extension appears to be a non-starter.

The supporters of simple extension, a politically mixed bag of liberals and conservatives, dominated the recent House hearings on the subject. Some legislators who had declared their skepticism about maintaining strict Justice Department oversight of electoral procedures in states covered by the act -- now disproportionately in the South because of historic interference there with blacks attempting to exercise their franchise -- have wavered after listening to the arguments of those persuaded of the measure's continued necessity, especially in the South.

If Mr. Reagan required further evidence of this support, he received it last week from an unlikely quarter. In Mississippi's Fourth Congressional District, Democrat Wayne Dowdy edged out his Republican opponent in a closely contested special election for a seat previously held by Republican Jon C. Hinson, who had resigned after being charged with attempted sodomy. Understandably, the Republicans chose to deny that Mr. Dowdy's 1,000-vote margin in a district solidly Republican for almost a decade marked a turning point for Democratic fortunes elsewhere. Instead, they stressed the local factors -- for example, voter response to Mr. Hinson's difficulties. Also, Republicans noted that -- at least on economic questions -- while Mr. Dowdy attacked the adminstration's proposed cuts in Social Security, at the same time he avoided attacking the entire Reagan economic program and even reminded voters regularly that he intended to keep independent of the Democrats' national leadership.

But what has last week's contest to do with the political price of voting rights? This : the voters of Mississippi's Fourth District made it plain that they favored an unrestricted extension of the Voting Rights Act. First during the Democratic primary and then in the special election campaign, Mr. Dowdy alone took the incautious position that the act should be extended in its present form without further delay.A substantial majority of the district's black voters endorsed him (45 percent of the Fourth District's population is black). They were joined by many white voters in the district, which includes much of the state's urban, business-oriented and professional classes on whose support the resurgence of Republicanism in the region has been based.

Southern Republican state charimen, who called weeks ago for renewing the Voting Rights Act, appear to have heard the message long before Mississippi voters drove it home with Mr. Dowdy's election. The future of their party in the South may depend upon Mr. Reagan's ability to recognize its newly evolved biracial electorate as it strives to eliminate the residue of racism from political life. In filing its initial Voting Rights lawsuit, the administration in turn may be testing for the presence of substantial southern or congressional opposition. If, however, there emerges no groundswell of complaint, perhaps the possibiliyy exists for the measure's surprisingly early and unabrasive extension. This would please most of the parties directly concerned -- including the southern Republicans.