10. Is there a ban on donations from corporations with government contracts to candidates?

Source 1: Articles 63(1), 64(1), and 67, Political Parties Act 2013.

Artículo 63 inciso 1°.- Los partidos políticos podrán recibir financiamiento privado, proveniente de personas naturales o jurídicas, dentro de los límites y con arreglo a los requisitos y condiciones establecidas en la presente Ley.

Artículo 64 inciso 1°. – Las contribuciones privadas solo pueden acreditarse a favor de los partidos políticos.

Artículo 67.- Los partidos políticos no pueden recibir contribuciones de: 
a. Cualquier entidad de derecho público o empresa de propiedad del Estado o con participación de éste; 
b. Instituciones religiosas de cualquier denominación; 
c. Partidos políticos y agencias de gobiernos extranjeros; 
d. Gremios y sindicatos; 
e. Personas naturales que se encuentren cumpliendo sentencias por delitos de corrupción, o cualquiera de los establecidos en la Ley Contra el Lavado de Dinero y de Activos, y en la Ley Contra el Crimen Organizado y Delitos de Realización Compleja.

Article 63, paragraph 1.- Political parties may receiv r private financing from individuals or legal entities, within the limits and in accordance with the requirements and conditions established in this Law.

Article 64, paragraph 1.- Private contributions may only be credited to political parties.

Article 67.- Political parties may not receive contributions from: 
a. Any public law entity or company owned by the State or in which the State has a stake; 
b. Religious institutions of any denomination; 
c. Political parties and agencies of foreign governments; 
d. Trade unions and labor unions; 
e. Individuals who are serving sentences for crimes of corruption, or any of those established in the Law Against Money Laundering and Assets, and in the Law Against Organized Crime and Complex Crimes.

Source 2: Article 10, paragraph 6, Provisions for the Nomination of Non-Party Candidates in
the 2010 Legislative Elections.

Artículo 10 inciso 6°.- 
El origen y uso de los fondos empleados estarán sujetos a la fiscalización que la ley establezca.

Article 10, paragraph 6.- 
The origin and use of the funds employed shall be subject to the oversight established by law.

Ley de Partidos: https://www.asamblea.gob.sv/sites/default/files/documents/decretos/2E597B83-D1C9-4C91-A9C9-6EC8A44FBA0F.pdf  
Regulación candidaturas no partidarias: https://www.asamblea.gob.sv/sites/default/files/documents/decretos/45E06ECC-27EC-49EC-B5D5-5D29814BAD32.pdf  

There is no explicit prohibition on donations from corporations with government contracts to candidates.
For independent (non-party) candidates, Article 10(6) of the 2010 Decree on the Nomination of Non-Party Candidates states that “the origin and use of funds shall be subject to the oversight established by law.” At the time of adoption, no specific law regulated political finance, creating a legal gap. Since the enactment of the Political Parties Act (2013) and its regulations (2014), this Act has been interpreted as the law providing such oversight, filling the earlier gap.
Accordingly, independent candidates are considered subject to the same criteria on the origin, use, and supervision of funds as established for political parties. However, the Political Parties Act does not prohibit donations from corporations with government contracts.
For party candidates, the provision does not apply directly, since only political parties may receive private contributions under Article 64 of the same Act, and this law likewise does not restrict donations from corporations with government contracts.
In fact, by not regulating the financing of party candidates, the law creates a loophole that is exploited in practice by candidates to raise funds without being accountable to their parties or the electoral authority for the origin and destination of the funds received.