On the face of it, one might think President Reagan's popularity was slipping with Virginia's conservative congressmen. Just last week, the 10 Virginia congressmen, nine of them Republicans, turned in an overwhelming vote against the administration's $98.3 billion tax increase package.

But state political observers don't believe for a second the Virginia delegation's 8-to-2 vote against the tax bill signals any real weakening of support for Reagan. Rather, they say, it was a vote of pure political expediency by a staunchly pro-Reagan crowd up for reelection in November.

"It's kind of hard to campaign and win votes and raise taxes all at the same time," said an aide to one Virginia congressman. "It's hard to be a purist in this business, and politics is unfortunately a factor in everything."

Although the Virginia delegation consistently has supported Reagan's economic proposals, the proposed tax increases put them between the proverbial rock and a hard spot.

Going with Reagan meant angering the powerful Virginia tobacco lobby, which opposed the administration's plan to double the federal excise tax on tobacco. As representatives of a state that is one of the nation's leading tobacco producers, the Virginians were acutely aware of what that could mean.

The tobacco industry estimated the tax bill would cause a 4 percent drop in cigarette sales nationally. In Virginia, that translated to a $240 million loss in tax revenues, not to mention the elimination of numerous jobs.

In the final tally, tobacco states voted against the Reagan package more than 2-to-1, and the Virginia delegation made sure it was among those in the opposition column.

John Palafoutas, an aide to Richmond Republican Thomas J. Bliley, said, "He had some real problems [with the bill]. Phillip Morris is the biggest employer in the district outside of the government."

Voting with Reagan also meant angering the hundreds of thousands of federal workers in Virginia who opposed the new 1.3 percent Medicare tax the bill will impose on them.

Five of the 10 congressional districts in Virginia include at least 25,000 federal workers and retirees, and those workers, already angry about other recent pay and pension setbacks, were not hesitant about calling their congressmen in the harried days preceding last Thursday's vote.

"They contacted us every minute," said Dick Leggitt, an aide to Northern Virginia Republican Stanford E. Parris. "Our phones were ringing off the hook."

As if that were not enough, Virginia congressmen also worried that a vote for Reagan's tax package would put them in the position of flouting Virginia's time-honored tradition of disdain for unchecked government spending.

Stacks of letters, Mailgrams and petitions delivered to their offices last week made it clear Virginia voters, weaned on the balanced budget philosophy of the late Harry F. Byrd Sr., would not take kindly to such behavior.

With all of that in the wind, it wasn't surprising that only two Virginians stood with Reagan on the tax bill. And both, M. Caldwell Butler of southwest Virginia and G. William Whitehurst of Virginia Beach, are Republicans who have little to fear from the voters this year. Butler has announced he will not seek another term, and Whitehurst is running unopposed.

Leading the Republican charge against the administration proposal was Rep. Paul S. Trible Jr. of Newport News, the GOP nominee for the U.S. Senate seat left vacant by retiring Sen. Harry F. Byrd Jr.

Trible, whose recent campaign literature omits mentioning the Republican Party, said he voted against the tax bill because he feared it would slow economic recovery.

But many Republicans on Capitol Hill said it was equally likely that Trible, an unflagging champion of the administration's economic policies in his work on the House Budget Committee, voted as he did to show the public he is not a Reagan clone.

Voting with Trible to oppose the tax bill were Republicans Bliley, Parris, Frank R. Wolf of Northern Virginia, Robert W. Daniel of southeast Virginia, J. Kenneth Robinson of Winchester and William C. Wampler of southwest Virginia. One Democrat, Dan Daniel of Danville, also voted against the proposal.

In the U.S. Senate, both Virginians, Republican John Warner and Independent Harry F. Byrd Jr., voted against the tax bill.