Virginia Republican Party leaders are gleefully savoring what they expect to be a reelection romp by President Reagan and U.S. Sen. John W. Warner 13 days from now. The party rooms at the new Richmond Marriott are already booked.

John S. Alderson, the Roanoke insurance man who heads the Reagan-Bush campaign in Virginia, told reporters recently that he expected "a smashing victory" for Reagan. Gone are his summer-long warnings that apathy and overconfidence could wreck any victory margins. "This party is revved up," Alderson now says.

State GOP Chairman Donald G. Huffman, a Roanoke attorney, said this week that party polls show Warner with about 60 percent of the vote and running a percentage point or two ahead of Reagan. Those findings are consistent with a Richmond Times-Dispatch poll taken in mid-September.

Democrats, who concede they have uphill battles in promoting the Mondale-Ferraro ticket and Senate nominee Edythe C. Harrison, the former legislator from Norfolk, are gamely expressing their support for the top of the ticket, but the party enthusiasm is hard to come by.

This week, Harrison, who has struggled to gain the backing of Democrats for her admitted underdog candidacy, expressed her frustration during a campaign swing in Northern Virginia.

In a remark to reporters, Harrison groused that her fund-raising efforts were hampered by the initial reluctance of Gov. Charles S. Robb to back her. She said Robb's hesitation probably cost her at least $500,000 in contributions.

Harrison, who reported raising $378,000 at the end of September -- including $100,000 of her own money -- badly trails Warner in funds, who reported $1.9 million in contributions. He announced recently that he was cutting back on fund-raising to leave money for other Republican congressional candidates.

Robb, who has attempted to keep a low political profile this fall, made a telling remark himself when he officiated at the opening of the Mondale-Ferraro campaign headquarters in Richmond a few weeks ago.

The governor said he had not renewed an invitation, first offered during the Democratic convention in San Francisco, for Geraldine Ferraro to make an appearance in Virginia.

"I have not renewed the invitation," Robb told reporters. He said he has suggested that she campaign in states "where she can make a difference."

(Mondale himself never was expected to campaign in the state that last went for a Democratic presidential candidate, Robb's father-in-law Lyndon B. Johnson, 20 years ago.)

Robb, likewise, has characterized Harrison's chances as "an uphill battle," although he recently has praised her issue-oriented campaign that has blasted Warner and the Republicans for running up the nation's budget deficits.

While the Democrats are nervously eyeing Nov. 6, the Republicans are beginning to weigh the potential effect of the general election on next year's crucial statewide contests for governor, lieutenant governor, attorney general and the 100-member House of Delegates.

"Every time a person votes Republican in a given election, it helps us," said state GOP Chairman Huffman, whose party still ranks a pale second to the Democrats at the courthouse and statehouse levels in Virginia. "It becomes that much easier to do," Huffman said.

Huffman noted that five of the GOP's eight congressional candidates this year were once Democrats, examples of a decades-long trend for conservative Democrats to swing to the Republican Party.

Huffman told reporters in early fall that GOP canvassing showed that many state Democrats were strong supporters of Reagan but not willing to declare themselves as Republicans. He said he believed the gradual shift of conservative Democrats, many of whom switched to independent in the early 1970s, is continuing.

But he said he is not willing to predict that this year's elections will be a watershed for the state GOP. "We won't know until subsequent elections," Huffman said.