The approximately 65 million Democrats who voted for Hillary Clinton should pledge that in the future if a Republican wins the presidency with fewer votes than a Democrat for the third time in our era, we won't pay taxes to the federal government. No taxation without representation!
Twice in the past 16 years, a Republican candidate who finished second in the popular vote has won the presidency. This year, Donald Trump won the electoral vote with about 46% of the popular vote, while Hillary Clinton received about 48%. If the parties stay this evenly divided, another electoral mishap is more likely than not in the next 20 years.
Most Republicans are quite content with this system. Appeals to fairness have not persuaded them of the need to amend the Constitution to establish direct presidential elections, preferably with a runoff if no one wins 50% of the vote. Nor does the real chance that a Democrat could win the presidency with fewer votes than a Republican alarm them. Even the taunt, "Are you afraid of a direct election? Can't you win a straight-up vote?" doesn't faze them. Democrats must, therefore, pester Republicans where it hurts: the pocketbook.
Is signing a pledge to not pay taxes legal? Yes, if no overt act of conspiracy is involved, and the pledge itself is hypothetical. No one knows when or if it would be carried out.
A national movement not to pay federal taxes in the future would put Republicans on notice: they do not have the right to impose a hard-right, second-place presidency on a moderate nation every dozen or so years. If the Republicans won't help amend the Constitution so that America can resume being a democracy, then Democrats, lacking the representation that supporters of a future popular vote-winner ought to have in the executive branch, should not submit to paying taxes to the federal government.