This answer lists some of the statistics and metrics used to measure political participation.
First, let's define political participation:
Conventional participation: Activities that we expect of good citizens. For most people, participation occurs every few years at election time. People strongly committed to politics are more likely to participate on a regular basis.
Example: Conventional political participation includes voting, volunteering for a political campaign, making a campaign donation, belonging to activist groups, and serving in public office.
Voter participation's the most useful way to measure political participation, especially voter turnout.
In the 1976 presidential election, voter turnout dropped 1.6% which isn't really significant. However, the number of voters who voted for the Republican candidate decreased by approx. 8 million while those who voted for the Democratic candidate increased by approx. 11 million. This shows that there's a huge swing towards the Democratic Party which is largely attributed to the unpopularity of Nixon and unhappiness that Ford pardoned him.
According to the Voter Turnout and Congressional Change article by Pew Research Center, it mentioned that the Watergate scandal might have caused Republican voters to sit out in elections.
Another way to produce a big change in Congress is a “one-party collapse,” where a huge number of voters from one of the parties simply sit out the election. That is what happened in 1974, when the dispiriting backdrop of the Watergate scandal led to a nearly 3-million vote falloff in the Republican House vote from 1970. The Democratic vote grew by barely 1 million. But the GOP drop off was so severe that it cost the Republicans nearly 50 House seats.
As seen in the graph below, the number of Americans who identify themselves as Republicans started to decline from 1972 onwards while those who identify themselves as Democrats and Independents started to increase.
Donations seemed to have dropped for both parties in the 1976 presidential election. However, this might be due to new laws created by Congress in 1975 to regulate the finances of presidential campaigns.
There are very specific laws governing how a campaign itself can raise money, which are enforced by the Federal Elections Commission, created by Congress in 1975 as the enforcement/compliance mechanism for campaign finance laws. As laid down by the FEC, corporations may not contribute to campaigns or national party committees (i.e., the Republican Party), and individuals are limited to $2,500 per candidate. The laws are similar for contributing to national party committees, although the personal limit is much higher, (currently) $30,800. And no, these personal donations are not tax deductible.
Regardless, it does show that campaign donations for the 1972 presidential election decreased and it increased in subsequent elections. Below shows the amount raised in the 1968, 1972, 1976 and 1980 presidential elections, adjusted for inflation.
Hubert Humphrey: $77.1 million
Richard Nixon: $168.9 million
George McGovern: $166.1 million
Richard Nixon: $339.9 million
Jimmy Carter: $135.8 million
Gerald Ford: $145.6 million
Jimmy Carter: $137.6 million
Ronald Reagan: $162 million
Many of the changes in political participation don't seem to be permanent. Many of the statistics and metrics showed that participation seemed to rebound in the 1980 presidential election and beyond.