Johnson is impeached on May 16 or on May 26.
According to the Presidential Succession Act of 1792 President Pro Tempore Benjamin Wade is sworn as Acting President and have to call a special election no fewer than two months later.
So the 1868 election so anticipated in the middle of July.
Republicans had nominated Grant at their National Convention in Chicago on May 21. Probably Wade is able to use his new influence to win the VP nomination.
Democrats are forced to anticipate their National Convention in New York at least to early June: rushing to find a candidate, they can not wait until Seymour is persuaded to run and quickly nominate George Pendleton from Ohio, their frontrunner and 1864 VP candidate, and Francis Preston Blair Jr from Missouri as their ticket.
With few time and an arranged ticket Democrats lose clearly against Grant, losing also New York, New Jersey and Oregon, 257-37.
Grant is sworn as 18th President on March 4, 1869.
Without Colfax as VP Grant Administration would seem less corrupt and probably do better in 1872: he defeats Liberal Republican-National Democratic candidate Charles Francis Adams, after the former Minister to Britain has defeated editor Horace Greeley for nomination, mainly due strong support of Greeley for Johnson's impeachment, that was opposed by majority of Liberal Republicans (and of course Democrats). Grant probably swing Maryland und maybe Tennessee (if Adams is perceived as more Northern than Greeley), winning 306-46.
With less attention to corruption in 1876 Senator James Blaine from Maine can confirm his status of frontrunner and wins Republican Nomination. Then he faces New York Governor Samuel Tilden for Democrats and possibly wins narrowly, but not so narrowly as Hayes, probably avoiding the Compromise of 1877.