IN 50 STATES and the District of Columbia, both parties -- and their technically nonpartisan auxiliaries -- are down to the last days of registering new voters. It's important business. Republicans have been spending heavily to ensure that their enthusiasts will register now and vote in November. Ann Lewis of the Democratic National Committee has said her party's chances hinge on increasing the electorate from 87 to 100 million. Now evidence is emerging that even that large an increase may help the Republicans. Can that be so?

The old rule of thumb in politics is that increased registration helps the Democrats. Blacks, the poor and young people tend to be unregistered when an election year begins and to be heavily represented among new registrants; all three groups have been Democratic for mostof the last 50 years. Blacks and, to only a somewhat lesser extent, the poor are still Democratic in 1984. But in the polls, young Americans have been among the groups most heavily in support of Ronald Reagan.

Black registration has been rising significantly -- though no one has hard figures on just how much -- since 1980. Evidence is accumulating that new registration among whites might be helping the Republicans more than anyone expected when the year began. A Gallup poll conducted for the Joint Center for Political Studies showed newly registered blacks voting nearly unanimously for Walter Mondale, with newly registered whites going 68 to 26 percent for Ronald Reagan. Market Opinion Research, which works for Republicans, reports that Republican Party identification is up this summer -- a development which coincides with increased registration and which, if true, bodes well for the whole Republican ticket. Close to home, in Montgomery County, where almost all the elected officials are Democrats, Republicans outnumber Democrats among new registrants for the first time in a decade.

Democrats continue to hold a big edge among voters who first registered during the New Deal and during the days when memory of the Vietnam War draft was fresh. For this year's new voters, on the other hand, the contrast between the parties is not between FDR and Herbert Hoover or John Kennedy and Richard Nixon; it is between Jimmy Carter and Ronald Reagan, and this pairing does not necessarily work to the Democrats' favor. It's dangerous to extrapolate from a few months' figures to a whole generation, and it's risky to bet that all newly registered Republicans will end up marking the Republican column in November. But the registration figures in hand suggest that the stone the Democrats are trying to roll uphill this year may be heavier than expected.