Act of June 6, 1898, ch. 389, 30 Stat. 432, 55th Congress removed the parts of the 14th Amendment that kept ex-Confederates as non-citizens and gave them complete amnesty. Along with Johnson's December 25, 1868 universal pardon and amnesty that restored complete rights to all who participated in the rebellion including General Lee so he has been a full citizen since 1898.

http://memory.loc.gov/cgi-bin/query/D?rbpebib:13:./temp/~ammem_906T::

"The 14th amendment to the Constitution, ratified in July 1868, provided that no person could hold any civil or military office under State or Federal Government who had previously taken an oath to support the Constitution while holding such a position and hen had engaged in rebellion against the United States. The amendment provided that the Congress, by a two-thirds vote in both Houses, could remove this disability, and between 1868 and 1872 a number of ex-Confederates, were granted congressional amnesty. The General Amnesty Act of May 22, 1872 (17 Stat. 142), left only a few hundred under the disabilities imposed by the 14th amendment, but it was not until June 6, 1898 (30 Stat. 432), that general amnesty was made universal. Also important in restoring former Confederates to full rights of citizenship was an act of May 13, 1884 (23 Stat. 21), which provided that prospective Federal jurors and civil officers no longer had to swear that they had never engaged in rebellion against the United States."

http://www.tngenweb.org/civilwar/pardons/center.html
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David Upton