How do politicians pay for political campaigns? Who provides the money to pay for campaign expenses?

Using the personal funds of the candidate. When candidates use their personal funds for campaign purposes, they are making contributions to their campaigns. Unlike other contributions, these candidate contributions are not subject to any limits. They must, however, be reported. Eligible candidates in the presidential primaries may receive public funds to match the private contributions they raise. While a candidate may raise money from many different sources, only contributions from individuals are matchable; contributions from PACs and party committees are not. Under the presidential public funding program, eligible presidential candidates receive federal government funds to pay for the qualified expenses of their political campaigns in both the primary and general elections. ... Fund the major party nominees' general election campaigns (and assist eligible minor party nominees).

Ifound this on google “separation of powers. a fundamental principle of the united states government, whereby powers and responsibilities are divided among the legislative branch, executive branch, and judicial branch.” i hope this you for what your looking for