President Reagan had barely finished his nationwide appeal for support of his new tax bill, his image just a memory on the television screen in Rep. Stanford Parris' office, when the buttons lit up on the bright blue console on the congressman's desk.

"Hello, this is Congressman Parris. Can I help you?" the Northern Virginia Republican said in his most congenial manner.

"Oh, the White House is telling you to call, you mean in the speech? Uh-huh, you called the White House and they told you to call your congressman?"

The answer was "yes" from the caller, who said she supported the president. She was the first of a rash of constituents whose calls would drive home the dilemma for Parris in the coming vote in this election year.

If he votes in favor of the $98.3 billion tax bill, it will anger the more than 55,000 federal workers in his district who oppose the new 1.3 percent Medicare tax that the bill would impose on them.

If he votes against the bill, he will have abandoned the president who brought him to office on his coattails in 1980 and whose help he wants in November.

By 8:55 p.m., the count from the telephone calls was 10 for the bill, 10 against and one undecided -- a split reflective of Parris' own mental tug-of-war over the measure and that of every Washington-area member of the House.

Each is being pushed and pulled in opposing directions, with the pressure particularly severe because of the huge number of federal workers in their districts and the fact that those constituents, already aroused by other pay and pension setbacks, are only a local phone call away.

As of yesterday afternoon, less than 48 hours before the House is expected to vote on the matter, none of the legislators had decided which way to land on the issue that could be one of the biggest in the November elections.

"I've decided this thing four different times, and I still don't know. It's one of those tough calls," Parris said, minutes after Reagan appealed to voters to tell their representatives that they support the $98.3 billion tax bill.

Parris' Republican colleague in Northern Virginia, Rep. Frank Wolf, was in just as much turmoil. "I sat home last night and watched the president, then watched the Democratic response. I've read every article . . . " Wolf said during a break in House business yesterday. "But I just don't feel very good about the bill. I haven't made up my mind."

While the Virginians and their Maryland counterparts -- Democratic Reps. Michael Barnes and Steny Hoyer -- wrestled with their consciences, groups that would be affected by the bill made the tough decision even tougher. The National Association of Letter Carriers, for example, sent mailgrams to all 435 House members to let them know that a vote in favor of the bill would be considered "a wrong vote" when the union tallies up lawmakers' scores to hand out endorsements.

Postal workers number in the thousands in the four congressional districts immediately around Washington, and they are only a part of the 156,000 federal employes who reside in those districts and keep close track of how their representatives vote.

The four local members of Congress have worked hard for those workers' approval.

Now, their standing with those constituents could be threatened with a single vote for the tax bill, which for the first time would require federal workers to pay 1.3 percent of their salaries for Medicare, up to a maximum of $456. "It's the classic incumbent's dilemma," said one lobbyist for federal employes. "No matter how they vote, there's more than enough for opponents to shoot at."

Indeed, the measure covers a wide range of areas, raising money through higher excise taxes on cigarettes and telephone service, tax increases aimed at corporations and the rich and a variety of measures to stop tax cheating.

Maryland's Democratic legislators who were not elected on promises to cut taxes, are under less severe pressure than their Republican colleagues when it comes to voting on a tax increase. Still, Barnes and Hoyer are caught between the Democratic leadership's directive to vote for the bill and their federal constituents' concerns about the Medicare tax.

Calls to Hoyer's office were running about 2 to 1 against the bill; in Barnes' office, the split was right down the middle. Neither had decided how to vote on the bill, but they both were sent to Congress on wide margins of victory and seemed relatively unconcerned about the possible impact on their chances for reelection.

The Virginia Republicans, elected in the Reagan landslide by margins of less than 2 percentage points, appear to be in a different position.

But Wolf said he didn't care what his opponent made of the vote. Parris who is running against former congressman Herbert Harris whom he defeated by fewer than 1,100 votes, said he didn't think "this vote would make or break a successful campaign.

"But I'm agonizing," Parris added as he left his office in the darkened Cannon House Office Building late Monday night. "I wonder about the psychological impact of a tax increase. If we increase your taxes, are you more or less likely to go out and buy a car or build a house. That's my concern.

"By the end of this week, I'll have to decide, yes or no. Unfortunately, they don't have a voting button marked maybe."