The two Democratic pioneers of tax simplification spent last week winging around the country in a campaign-style blitz attempting to rev up public support, but they discovered the American people apparently aren't ready to barrage Capitol Hill with demands to close their loopholes and cut their rates.

Sen. Bill Bradley (D-N.J.) and Rep. Richard A. Gephardt (D-Mo.) found that the people with whom they had contact generally liked the idea of simpler and fairer taxes.

But their interest, considered essential to persuading a nervous Congress to overhaul the tax code, was more idle than ideological. And they have pet provisions they don't want to lose.

Passing a tax-simplification bill "will be the political war to end all wars," Gephardt said in Houston. "This gets into the knickers of everybody in the country."

The two Democrats also had another goal, to seize the political initiative on tax simplification before it belongs to the Republicans.

"The place Democrats have to be is 100 percent behind tax reform," Bradley told a Chicago audience. "We share the spotlight if the president comes forward and pushes it. If the Republicans in the Senate kill it or the president backs away, we then have an economic growth issue for a decade."

Questions from the news media, fellow Democrats and business people were more about specific tax loopholes than politics, however.

In Houston, the questions concerned tax breaks for the oil and gas industry. In St. Louis, Gephardt was asked about terminating the tax credit for rehabilitating historic buildings, which residents said had helped revive decaying neighborhoods, some in his district.

Nancy Sue Yawotiz, 25, looking approvingly at the 17-line simplified "tax form" Gephardt had given her as he worked the crowd at a farmers' market, said she might not mind seeing her fringe benefits taxed as income, which Bradley-Gephardt would do, but only if her rates were reduced.

Glen Robitsch, 41, was more blunt: "I don't like the idea of taxing my fringe benefits. I already pay enough."

"Loophole illusion," Bradley called it, the phenomenon of examining the tax-simplification proposal by looking only at specific changes rather than the plan's overall impact. Mostly he was asked about the impact of eliminating one break or another.

But for an issue that holds such potential for both political parties, Bradley found his audiences and the local news media lacking passion. His questioners displayed little emotion on the subject, while reporters expressed indifference.

"Our challenge this morning," Walter Jacobson, the anchorman at WBBM-TV in Chicago, told Bradley as they hitched up their microphones for an interview, "is to talk for 30 minutes about tax reform and have one viewer left."

Two-thirds through the taping, when the interviewers broke for a commercial, Jacobson sounded exhausted.

"I can't even think of one intelligent question to ask about the tax bill," he admitted. "It's not irresponsible to get into other subjects, is it? You do work in the Senate as a whole. Your interest isn't confined just to economic matters, is it?"

"I don't want to talk about anything else," Bradley deadpanned.

Bradley then suggested they talk about trade.

"Foreign trade?" Jacobson responded.

On its evening news shows, WBBM used two clips from the interview, which aired in full yesterday morning. One clip had to do with Bradley's interest in running for president.

The other was about basketball, Bradley being a former professional player.

Two years ago, Bradley and Gephardt were lonely voices in the movement to revamp the income-tax system. Last year, they tried to sell Walter F. Mondale on the idea for his presidential campaign, but he balked -- a move now viewed by many as a mistake with serious consequences for the Democrats.

At the time Mondale spurned the proposal, Republicans were onto the issue. Rep. Jack Kemp (R-N.Y.) and Sen. Robert W. Kasten Jr. (R-Wis.) had introduced a modified flat-tax proposal.

Then in November, President Reagan's Treasury Department unveiled its own tax-simplification plan. In February, the president embraced the concept in his State of the Union Address, helping to identify major tax reform as a Republican issue.

Saturday, Reagan said in his weekly radio address that he will offer the rewritten version of the Treasury plan shortly after his return from Europe in mid-May.

The House Ways and Means Committee plans to begin hearings almost immediately afterward, and Chairman Dan Rostenkowski (D-Ill.) said he hopes to send some kind of bill -- he has endorsed the idea of reform without specifying what kind -- to the Senate this year.

The issue has gathered an unusual political momentum, if not tremendous public interest. Both political parties are seeking to embrace tax simplification, if only to prevent the other from getting full credit if it passes.

"It's a bipartisan opportunity," Bradley told a television interviewer last week. "But let's be candid as well. If the president comes out and really does go on TV and backs fundamental tax reform that's meaningful . . . , it's a little bit like a whale jumping in the pool. There's less water to swim in."

From the dawn-to-dusk schedule to the multiple local-media appearances to the Mondale campaign refugees handling the advance work, the cross-country trip had the look of a budding political campaign. Bradley said he wasn't running for president, but Gephardt hedged.

Gephardt asserted repeatedly that he was not thinking about the 1988 presidential race and is busy with his new job as chairman of the House Democratic Caucus. Asked why he doesn't just rule out a presidential race, like Bradley, and end the speculation, Gephardt said, "You never rule anything out."

The trip was full of the political implications of tax revision, too. Bradley argued that self-interest compels the Democratic Party to work for tax simplification, and he suggested that it still is possible for the Democrats to reap much of the credit. "It is one of those times where there is a congruence of party interest," he told the Greater Miami Chamber of Commerce.

"It allows you to go to the Democrats who voted for Ronald Reagan in 1980 and 1984 for what I call . . . feel-good reasons . . . with an issue that is in their direct economic interest: lower tax rates," he said. "That is the stuff of realignment. It's clear what's in it for Republicans, if they support it and the president pushes for it."

Fellow Democrats were a bit dubious, however. Party officials at a St. Louis meeting asked Gephardt about numerous deductions that would be terminated. In Houston, Mayor Kathy Whitmire said she wasn't aware of a consensus on how to change the tax laws, and Harris County party chairman Barbara Stanley said that at the moment, people are "fairly negative" about such plans as Bradley-Gephardt.

Bradley and Gephardt planned to close out their media blitz by passing out their "Form 1040 BG" at the central post office in New York City today to catch the last-minute tax-filing crowd.

Gephardt, asked if the effort would change many minds, responded by citing his Missouri mentor, former representative Richard Bolling: "The main determinant of members' votes is what they hear from people at home.

"I have found it doesn't take a lot of phone calls and letters to make members think there's a groundswell," Gephardt said. "You just need enough. If nobody shouts, obviously we'll be outgunned. But if there can be a steady flow, enough phone calls and letters -- I think there is intensity."